Allegedly, on or about 27 November 2013, AP sent:
Well, I am replying from Opera browser right now.
And the headers are still missing.
What happens if you don't block scripting while you're doing this? Enable it before you hit reply, type your reply, then send it.
My thinking reckons that if it still behaves oddly when you use an unmodified browser, it's a gmail fault.
I couldn't see any options in the gmail settings that might hide such information. You're not using one of those "private browsing/incognito window" functions of the web browser? It's conceivable that gmail might take notice of such a thing, and obscure some of the mail headers. Not that it'd do you much good, since you still have your own address in the reply, anyway.